This invention relates to drilling of wells and particularly to drilling fluids and their use to allow drilling through shale formations.
Drilling of shale is a very old problem. One review of the U.S. patent literature indicates that about 800 different substances have been disclosed for drilling shales in the patents issuing prior to 1954. Flaxseed gum (U.S. Pat. No. 1,807,082), gilsonite (U.S. Pat. No. 2,812,161), and potassium chloride (U.S. Pat. No. 1,460,788) are among the various additives used for this purpose.
The technique of using flaxseed gum and the technique of using potassium or ammonium salts are both old. There are some troublesome shales which neither of these techniques by itself allows to be drilled effectively.
In U.S. Pat. No. 3,017,351 issued to Scott, Anderson, and Park on Jan. 16, 1962, the use of at least 3 pounds per barrel of water-soluble ammonium and potassium salts is taught to prevent swelling of shale and to decrease the tendency of shale to slough in clear-water drilling. That patent also teaches the use of a water-soluble amine and an acrylamide polymer hydrolyte to provide a coating of a protective film on the surface of the shale to prevent softening and breaking of the shale by mechanical action. The use of such combinations of chemicals in a clay-based mud, however, is not practiced because the inorganic salts involved would flocculate the clays being used for viscosity and fluid loss control, resulting in undesirably thick muds, high fluid loss, and thick filter cake which tends to stick the drill string.
The traditional method of drilling with a saltwater fluid or with any kind of salt contamination was to formulate an attapulgite clay-based mud using starch as a fluid loss additive, but such a mud resulted in very slow drilling rates. U.S. Pat. No. 3,723,311, issued to Lummus and Edwards on Mar. 27, 1973, teaches an alternate saltwater drilling technique using a non-clay based mud containing flaxseed gum, gilsonite, and asbestos. The viscosity, fluid loss, and filter cake thickness of the fluid of that patent are unaffected by salt content and faster drilling characteristics were obtained. That patent does not teach, however, that any particular combinations of salts are necessary for shale stabilization, but, on the contrary, states that water-soluble salt content may be anywhere from zero to saturation. The only use of salt taught by that patent is the use of saturated salt solution to prevent enlargement of the borehole by preventing the dissolving of drilled salt sections, which are, of course, mostly sodium chloride.
Despite the large number of substances which have been used in drilling through shale sections, certain shales are still encountered which none of the prior art practices allow to be drilled effectively. The Montrose shale, which is being encountered in the North Sea, is an example of such a shale. With shales of this kind, difficulties, such as sloughing hole and sticking of the drill bit, continue to be encountered.
When such shales are drilled by conventional techniques, additions to maintain the shale-stabilizing drilling fluid must be continued to be made while drilling below the shale formation as (unless an additional casing is run) the drilling fluid is still in contact with the shale formation.